1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to a method of controlling interference in a wireless communication system.
2. Discussion of the Related Art
Multi-Input Multi-Output (MIMO) technology improves efficiency of data transmission and receipt using a multiple transmission antenna and a multiple receipt antenna.
While a receiver receives data through a single path if a single antenna is used, a receiver receives data through multiple paths if a multiple antenna is used. Thus, speed and capacity of data transmission is improved and coverage is enlarged.
Generally, there are an open-loop multiple antenna system in which a transmitter doesn't use feedback information from a receiver and a closed-loop multiple antenna system in which a transmitter uses feedback information from a receiver in multiple antenna systems. In a closed-loop multiple antenna system, a receiver transmits feedback information about channel status to a transmitter and the transmitter detects channel status through it so that performance of a communication system is improved.
A transmitter multiplies data by precoding matrix and transmits the data. Thus, transmission signal (y) which is transmitted through a multiple antenna by a transmitter is represented as Equation 1.y=Wz  [Equation 1]
Herein, W is an element of a predetermined codebook, and z is a signal outputted from a MIMO encoder. z is as equation 2 when it is encoded with a spatial frequency block coding (SFBC) and a spatial time block coding (STBC). And, z is as equation 3 when a MIMO encoder uses spatial multiplexing or rank and stream is 1.
                    z        =                  [                                                                      s                  1                                                                              -                                      s                    2                    *                                                                                                                        s                  2                                                                              s                  1                  *                                                              ]                                    [                  Equation          ⁢                                          ⁢          2                ]                                z        =                  [                                                                      s                  1                                                                                                      s                  2                                                                                    ⋮                                                                                      s                  R                                                              ]                                    [                  Equation          ⁢                                          ⁢          3                ]            
In equations 2 and 3, indexes of rows are stream indexes. Indexes of columns are subcarrier indexes in case of space frequency block coding, and indexes of columns are time indexes in case of space time block coding.
In a closed-loop multiple antenna system, a receiver selects a precoding matrix from a predetermined codebook and transmits an index of the selected precoding matrix to a transmitter. Then, the transmitter multiplies data by the selected precoding matrix and transmits the data. In a open-loop multiple antenna system, a receiver does not transmit a feedback for a precoding matrix, and a transmitter precodes data using precoding matrixes of a predetermined codebook rotationally.
However, when precoding and/or beamforming technology is used, a precoding matrix or beamformer may give interference to neighbor cells. Signal receipt performance of a receiver is depreciated because of the interference.